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threaded steel suction line

I had a customer today that wanted me to make a copper to steel transition .

This is for a rig to cool natural gas so they can crack it.

Evap temp is gonna run -10f

They wanted to run the suction line in schedule 80 threaded pipe . I said not a good idea ,may hold at first but after cycling the machine I was afraid thermal expansion and contraction would cause leaks . They are now going to socket weld it . They wouldn't let me run copper as they already have pipe and fittings and say they are close to losing money on this deal.

And lots of small coolers use steel discharge lines, condensers, and
receivers. Older stuff mostly, but its been done for years.

This is true, along with the fact that most compressors and filter driers are also steel. With that being said, though, I would think regular black pipe would be way to dirty. Also, I would think it would rust out very easily. A suction line is not the same as a discharge line, condenser, or receiver in this regard.

I would try as hard as possible to run away from this job, or at the very least have them sign a waiver that says they recognize that it isn't recommended and they will assume all risks if it doesn't work out as well as they hope.

If they are trying this hard to save money, it doesn't seem like it would end well for you to be in the middle of it.

Edit...

I just re-read your post. In it you say "This is for a rig to cool natural gas so they can crack it". I don't understand what you mean by this, but if they are just looking for a way to be able to take apart the piping easily, you can get solder type brass unions. I have never seen them used for refrigeration before, but it would probably be better than steel.